The other day the BBC's Parliament channel screened old coverage of the February 1974 election. There was a laconic press conference with a pipe-smoking Harold Wilson in his Huyton constituency. "Looks like the Tories will get more votes, but Labour more seats," said a journalist. "That can't be fair?"

"Other way round in 1951," replied Wilson, putting his pipe back in his mouth.

And that's still the truth of the British system today. Twice since the second world war the winning party in votes has been the losing party in seats, and the prime minister's job. Labour in 1951 actually got more votes than ever before or since – 1.3m more than the Conservatives, who took power.

Today's Guardian/ICM poll shows something like it might happen again next month. If it is right – and other polls this morning put the gap between the two main parties at 10 points, not ICM's four – then Labour has a chance of winning by losing. If votes shift equally in every seat – so called uniform national swing – then Labour would probably end up with more MPs, but substantially fewer votes. True, marginals may go more heavily Tory and so push David Cameron into No 10. But the party can't count on that.

It wouldn't matter that the Tory lead in votes, even in the ICM poll, is actually larger in the poll than between Tony Blair and Michael Howard in 2005. That's the system. David Cameron could protest. He could call for new parliamentary boundaries. He could try trigger a second election. But he'd be on the opposition benches – and that's what counts.